In recent years, healthcare services and life care services have attracted much attention. A Wireless Body Area Network (hereinafter referred to as WBAN) capable of supporting both medical and consumer electronics (hereinafter referred to as CE) services has become the next generation wireless technology for wireless personal area networks (WPAN). A WBAN composed of a coordinator, medical devices and CE devices may provide various services in medical and non-medical domains. At the early stage of WBAN development, the IEEE 802.15.4 protocol, which is a representative low-power protocol for WPANs, was considered for WBAN services. The reason for considering IEEE 802.15.4 for WBAN is that IEEE 802.15.4 provides low data rate, narrow transmission range and low power consumption, which are common to the requirements of IEEE 802.15.6 The IEEE 802.15.4 media access control (MAC) protocol provides three modes of frame structure such as beacon-enabled superframe, beacon-enabled non-superframe and non-beacon mode. In IEEE 802.15.4, both beacon-enabled non-superframe mode and non-beacon mode operate only on a contention basis. Meanwhile, IEEE 802.15.4 with beacon-enabled superframe mode includes an active period and inactive period. The active period is divided into 16 equal-sized slots, and includes a contention-free period (CFP) and contention-access period (CAP). In particular, the inactive period may be used to reduce power consumption. However, as IEEE 802.15.4 with beacon-enabled superframe mode requires additional time synchronization overhead, only IEEE 802.15.4 with both beacon-enabled non-superframe and non-beacon mode is actually used in the field.
Meanwhile, the IEEE 802.15.6 task group has established WBAN standardization. The objective of the IEEE 802.15.6 task group is to standardize the PHY and MAC protocols for WBANs, which can provide various ubiquitous services. According to the draft document published in March of 2011 by the IEEE 802.15.6 task group, to provide the PHY layer, the WBAN MAC protocol has a hybrid superframe-based structure composed of CAPs and CFPs. We focus on channel access in the CAP because only contention-based channel access is actually used in the field, similarly to the case of IEEE 802.15.4.
In general, the CAP consumes more energy than the CFP because nodes operate on a contention basis. However, as described above, the CFP is not used in the field. Focusing on the CAP, we can consider the following scenario. When many nodes are densely deployed in a narrow region, contention complexity is increased, resulting in high power consumption and numerous collisions. Particularly, in a WBAN environment requiring ultra-low power consumption, low contention complexity can help to reduce power consumption. One WBAN should have up to 256 nodes within 3 to 5 meters of the human body, and up to 10 WBANs must coexist in a space of 6×6×6 m3 with fair bandwidth sharing. In this sense, contention complexity, the number of collisions and power consumption should be increased. Therefore, contention complexity of WBAN is one of the most important keys to satisfaction of WBAN requirements.
Meanwhile, the WBAN draft document provides eight priorities for various packets such as background, best effort, excellent effort, video, voice, medical data or network control, high-priority medical data or network control, emergency or medical event report. To satisfy QoS requirements of WBANs, each priority has distinct maximum/minimum contention windows and contention probability values to realize priority-based channel access.
However, like IEEE 802.11, WBAN contention complexity may cause numerous collisions and more power consumption because up to 256 nodes can join a single WBAN and nodes are densely deployed in a narrow region. Therefore, the priority-based channel access policy defined in the WBAN draft document may also fail to resolve the fundamental contention complexity problem.